Let’s go in detail about the pollutants emitted from Spark Ignition (SI) and Compression Ignition (CI) engines — what they are, how they form, and their effects on the environment.
In this article:
- 1. Basic Difference Between SI and CI Engines
- 2. Major Pollutants from Automobile Engines
- 3. Pollutants from SI (Spark Ignition / Petrol) Engine
- 4. Pollutants from CI (Compression Ignition / Diesel) Engine
- 5. Comparison Table — SI vs CI Engine Pollutants
- 6. Control Methods for Engine Emissions
- 7. Environmental Impact Summary
- 8. Summary
1. Basic Difference Between SI and CI Engines
| Engine Type | Full Form | Common Fuel | Ignition Type |
|---|---|---|---|
| SI Engine | Spark Ignition Engine | Petrol (Gasoline) | Spark plug ignition |
| CI Engine | Compression Ignition Engine | Diesel | Self-ignition due to compression |
Because of their different combustion methods and fuels, the type and amount of pollutants they produce also differ.
2. Major Pollutants from Automobile Engines
Both SI and CI engines emit these main pollutants:
- Carbon Monoxide (CO)
- Unburnt Hydrocarbons (HC)
- Oxides of Nitrogen (NOₓ)
- Carbon Dioxide (CO₂)
- Particulate Matter (PM or soot)
- Sulfur Oxides (SOₓ)
Let’s look at each pollutant in detail, and how it differs for SI and CI engines.
3. Pollutants from SI (Spark Ignition / Petrol) Engine
1️⃣ Carbon Monoxide (CO)
- Source: Incomplete combustion of petrol due to lack of oxygen (rich air–fuel mixture).
- Reason: SI engines often operate at near-stoichiometric ratios, so incomplete burning can occur at idle or acceleration.
- Effect:
- Toxic gas; binds with hemoglobin and reduces oxygen in blood.
- Causes headaches, dizziness, even death in high concentrations.
➡️ High in SI engines, lower in CI engines.
2️⃣ Unburnt Hydrocarbons (HC)
- Source: Fuel that escapes unburned from the cylinder (leakage, misfire, quenching near walls).
- Reason: Poor mixture distribution and incomplete flame propagation.
- Effect:
- Contributes to smog formation.
- Some HC compounds are carcinogenic.
➡️ High in SI engines, lower in CI engines (due to higher combustion efficiency).
3️⃣ Oxides of Nitrogen (NOₓ)
Source: High combustion temperatures and oxygen cause nitrogen in air to oxidize.
Formation:
Effect:
Causes acid rain, respiratory irritation, and smog.
➡️ Moderate in SI engines, higher in CI engines due to higher combustion temperature and pressure.
4️⃣ Carbon Dioxide (CO₂)
- Source: Complete combustion of fuel.
- Effect: Greenhouse gas → contributes to global warming.
➡️ Emitted by both SI and CI engines (proportional to fuel burned).
5️⃣ Particulate Matter (PM / Soot)
- Source: Very small carbon particles due to incomplete combustion.
- Effect:
- Respiratory and cardiovascular diseases.
- Environmental pollution (smoke and haze).
➡️ Very low in SI engines, very high in CI engines (diesel smoke).
6️⃣ Sulfur Oxides (SOₓ)
- Source: Sulfur impurities in petrol or diesel fuel.
- Effect:
- Contributes to acid rain and corrosion.
➡️ Higher in CI engines because diesel usually contains more sulfur than petrol.
4. Pollutants from CI (Compression Ignition / Diesel) Engine
| Pollutant | Level | Reason |
|---|---|---|
| CO (Carbon Monoxide) | Low | More air → lean mixture → more complete combustion |
| HC (Unburnt Hydrocarbons) | Low–Moderate | Some unburned fuel due to poor atomization |
| NOₓ (Nitrogen Oxides) | High | High temperature and pressure promote NOₓ formation |
| CO₂ (Carbon Dioxide) | Moderate | Depends on fuel efficiency |
| Particulate Matter (PM) | Very High | Incomplete combustion → soot and smoke |
| SOₓ (Sulfur Oxides) | High | Diesel contains more sulfur than petrol |
5. Comparison Table — SI vs CI Engine Pollutants
| Pollutant | SI (Petrol Engine) | CI (Diesel Engine) | Remarks |
|---|---|---|---|
| CO (Carbon Monoxide) | High | Low | Due to rich mixture in SI engines |
| HC (Unburnt Hydrocarbons) | High | Low–Moderate | SI engines have incomplete burning at times |
| NOₓ (Nitrogen Oxides) | Moderate | High | CI engines operate at higher temperature and pressure |
| CO₂ (Carbon Dioxide) | Moderate | Moderate | Depends on fuel used |
| Particulates (Soot) | Very Low | High | CI engines emit soot due to incomplete combustion |
| SOₓ (Sulfur Oxides) | Low | High | Diesel has more sulfur content |
6. Control Methods for Engine Emissions
For SI Engines:
- Catalytic Converter: Converts CO, HC, and NOₓ into CO₂, H₂O, and N₂.
- Proper Air–Fuel Ratio: Stoichiometric ratio (14.7:1) ensures complete combustion.
- Electronic Fuel Injection (EFI): Improves fuel atomization and control.
For CI Engines:
- Exhaust Gas Recirculation (EGR): Reduces NOₓ by recirculating part of exhaust gases to lower combustion temperature.
- Diesel Particulate Filter (DPF): Captures and burns soot particles.
- Selective Catalytic Reduction (SCR): Converts NOₓ into harmless N₂ and H₂O using urea (AdBlue).
- Use of Low-Sulfur Diesel: Reduces SOₓ emissions.
7. Environmental Impact Summary
| Pollutant | Environmental Impact |
|---|---|
| CO | Toxic, reduces oxygen in humans and animals |
| HC | Forms smog, carcinogenic |
| NOₓ | Acid rain, smog, respiratory diseases |
| CO₂ | Greenhouse gas (global warming) |
| Particulates (PM) | Air pollution, health hazard |
| SOₓ | Acid rain, plant damage |
8. Summary
| Engine Type | Major Pollutants | Key Characteristics |
|---|---|---|
| SI (Petrol) | CO, HC, NOₓ | Clean exhaust but more gaseous pollutants |
| CI (Diesel) | NOₓ, PM, SOₓ | Higher efficiency but more visible smoke |
| Both | CO₂ | Greenhouse gas from fuel combustion |
In Short:
- SI Engines → Emit more CO and HC (due to incomplete combustion).
- CI Engines → Emit more NOₓ, PM, and SOₓ (due to higher temperature and sulfur content).
- Both contribute CO₂, which is unavoidable in fuel combustion.
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